1. Field of the Invention
The present invention relates to a method for producing a composite insulator and a retaining ring for retaining a core rod on a molding die. Particularly, the present invention effectively prevents generation of cracks on a parting line portion formed at a molding die corresponding to a dividing position of the molding die at the time of molding a sheath and a shed portion using a pair of divided molding die portions. The term "insulator" used herein means also a so-called "hollow insulator".
2. Related Art Statement
Heretofore, a composite insulator having a solid or hollow core rod made of a fiber-reinforced resin or the like and an insulative high molecular material applied on the outer cicumferential surface of the core rod to integrally form a sheath and a plurality of shed portions separately disposed in the axial direction of the core rod is known. Such a composite insulator is produced mostly by compression molding, injection molding, transfer molding or the like of an insulative high molecular material using a pair of molding die portions defining a cavity therein.
When producing such a composite insulator by vulcanization molding of an insulative material under heating and pressure filled in the cavity of the divided molding die portions, there arises a high concern that cracks are generated along a parting line on the sheath and shed portions of the composite insulator at a portion corresponding to a dividing position of the molding die portions due to thermal expansion of the core rod positioned at a determined position in the cavity of the molding die portions, particularly if the core rod has a large diameter of at least about 40 mm.
Such a phenomenon is considered to occur due to concentration of stress at the parting line portion positioned corresponding to the dividing position of the molding die portions at an instance of releasing the molding die portions for releasing the molded article from the molding die portions, when an inner pressure in an insulative high molecular material is increased to a certain level or more due to proceeding of thermal expansion of the insulative high molecular material heated in the cavity of the divided molding die portions.
Thus, the cracks on the parting line portion can effectively be prevented from occurring by restricting the increase of inner pressure in the insulative high molecular material caused by thermal expansion of the core rod, even though the thermal expansion of the core rod per se is unavoidable.
Then, in order to prevent the occurrence of the aforedescribed cracks, there are proposed a way of providing on the molding die portions a number of outlet grooves for allowing the high molecular material to escape in the grooves even when the core rod is thermally expanded so as to restrict the increase of the inner pressure in the insulative high molecular material in the cavity, a way of reducing a tightening force for combining the pair of the molding die portions together to allow a relatively large amount of the high molecular material flow in the cavity defined by the combined molding die portions at the time of vulcanization forming of the molecular material to restrict the increase of the inner pressure in the high molecular material, and a way of reducing the extent of the thermal expansion of the core rod by setting a temperature for heating the molding die portions at low levels to decrease the inner pressure in the high molecular material, and the like.
However, in the conventional way of providing a number of escaping grooves on the molding die portions, many processing treatments on the opposing surfaces of the pair of the molding die portions are required which result in a disadvantage of necessitating a large number of processing processes for the molding die portions and a disadvantage of necessitating a large number of working processes, such as cleaning of the molding die portions and the like, at each time of completing the molding operation. Also, in the conventional way of reducing a tightening force for combining the pair the molding die portions together, particularly in the case of pressing molding, a disadvantage occurs sometimes in that the products of desired shape cannot be obtained. And, in the conventional way of setting a heating temperature for the molding die portions at low levels, there is a disadvantage in that the time for the vulcanization heating treatment is unavoidably prolonged to increase the cycle time to decrease the production efficiency.